Polyolefin films are commonly used in the food and consumer product packaging industry, due at least in part to their numerous favorable properties related to this purpose. Such polyolefin films frequently are multi-layer films having a core, typically comprising polypropylene, and one or more layers of the same or different polymeric materials on either or both sides of the core. Polyolefin films of this type, however, have some limitations or drawbacks that have hampered, to a degree, the realization of their full potential as packaging materials for food products and especially perishable food products. One such limitation sometimes encountered is their lack of adequate resistance to the transmission of oxygen and/or water vapor, i.e., moisture.
An approach for improving the oxygen and/or water vapor transmission performance of polymeric films, such as those to be used for food packaging, involves the application of a barrier coating, such as a metal coating, to a polymeric surface of or in the film structure, to thereby form what may be described as a metallized film. Multi-layer polymeric films having a thin metal layer deposed on one or more of the polymeric layers of the film are known in the industry.
Metallized oriented polypropylene (OPP) films are frequently used as packaging for foods such as potato chips, crackers, and other fat-containing snack foods. The metallized films used for such comestible products commonly are laminated to film structures, such as structures that may generally be reverse printed. The lamination of the metallized film with a printed transparent film may provide a composite film structure with suitable barrier properties that also protects the ink and the metal layers within the laminated layers of the composite film structure. This lamination of metallized OPP films with a protective substrate is generally carried out via an adhesive lamination process or via an extrusion lamination process. In the case of extrusion lamination, the metallized multi-layer film being laminated has to withstand a rather significant thermal shock due to the contact with a hot, melted bonding-polymer. This thermal shock can lead to metal “crazing”, i.e., microcracking of the metal, e.g., aluminum, layer within the film. Metallized films used in the preparation of laminated film structures that are prepared in this manner should therefore exhibit a combination of good barrier properties with good metal adhesion after lamination and must have adequate resistance against crazing of the metal layer which might tend to arise as a result of the extrusion lamination conditions.
One way of improving the adhesion and crazing resistance of metal deposited onto copolymer skin layers of multi-layer polyolefin films involves some surface treatment of those skin layers prior to deposition of the metal thereon during film preparation, as discussed previously herein. Surface treatment which favorably alters the surface tension characteristics of such copolymer skin layers to improve metal deposition thereon and metal adhesion thereto includes flame treatment, corona discharge treatment and/or plasma treatment.
Technological improvements in plasma treating equipment, including reduced costs and improved efficiencies related thereto, have made this treating technology more commonly used in a variety of applications, including applications such as in-vacuum chamber treatment of film layer surfaces prior to metallization. In some instances, this technology may facilitate some advantages as compared to untreated or otherwise treated, metallized OPP substrates, such as lower oxygen and moisture permeability, a more consistent barrier, and improved metal adhesion strength.
Film layer pretreatment of certain types of polymeric substrates under certain conditions may, however, also adversely affect other properties, such as adhesion of ink, adhesives, and also metal to the surfaces so treated. For example, propylene and butylene co-monomers in treated films can undergo β-scission under flame, corona, or plasma treatment. Such β-scission may generate low molecular weight oxidized materials on the film surface. A certain portion of these low molecular weight oxidized materials are not well anchored at the film surface and significant concentrations of these easily removable low molecular weight oxidized materials may cause problems with respect to reduced adhesion of ink, adhesives, and metals subsequently deposited on these treated surfaces. The proportion of the problematic, easily removed, low molecular weight, oxidized materials at the film surface is referred to as the “labile oxygen ratio” at the film surface and may be expressed as a percentage of the total weight of the oxidized material on or near the treated film surface.
Considering the competing phenomena which occur during film layer treatment prior to metal deposition and film lamination, it would be desirable to identify combinations of film materials and surface pretreatment procedures and conditions that can facilitate preparation of especially useful metallized films having improved performance properties, such as barrier properties and metal and ink adhesion properties. In particular, it would be advantageous to identify and control the film elements and film preparation elements that can maximize the beneficial effects of pre-metallization, film surface pretreatment, with respect to the ultimate barrier, metal adhesion, and metal crazing properties of the films. It may also be advantageous to understand how to control such elements to simultaneously minimize any detrimental pretreatment effects, which can also occur during the preparation and use of these metallized polymeric films.
A number of prior art references describe preparation of metallized polymeric films with copolymeric skin layers having a metal layer deposited thereon. Such films have been prepared using surface pretreatment, including plasma treatment, of polymeric copolymer layers onto which metal is to be deposited. Representative prior art patents and applications relating to this type of technology are summarized as follows:
Nassi et al; European Patent Application No. EP-A-1,634,699; published Mar. 15, 2006 discloses production of multi-layer metallized films which include a plastic layer onto which a metal layer is deposited by vacuum evaporation. The plastic layer onto which the metal layer is deposited can comprise propylene-butylene copolymers. This plastic layer is pretreated using a combination of both flame treatment and plasma treatment prior to deposition of the metal layer.
Cretekos et al; U.S. Pat. No. 6,773,818; issued Aug. 10, 2004 discloses an oriented multi-layer film containing a core layer and a first skin layer which is metallized. The first skin layer must contain a metallocene-catalyzed propylene homopolymer or copolymer. The copolymer can include propylene-butylene copolymers or ethylene-propylene-butylene terpolymers. Exemplified are films wherein the first skin layer, prior to vacuum metallization, is subjected to varying types and degrees of plasma treatment. Such films can be laminated with a variety of protective substrates.
Scarati et al; European Patent No. EP-B-787,582; granted Jun. 4, 2003 discloses metallized polypropylene films which can be printed or laminated and which exhibit improved adhesion of the metal layer therein. Such films are formed by depositing metal onto a surface-treated copolymeric surface layer which can include copolymers of propylene with butylene and/or ethylene and which are treated to have minimized amounts of waxy oligomeric additives or residues left over from the polymerization process.
Fatica et al; U.S. Pat. No. 6,033,786; issued Mar. 7, 2000 discloses biaxially oriented, multi-layer films comprising a core layer and a bonding layer which has a flame-treated surface onto which a metal coating can be deposited. The bonding layer can comprise propylene/butene-1 copolymers containing up to 14 wt % butene. This layer is flame-treated prior to metal deposition thereon with this flame treatment said to provide superior adhesion of the metal in comparison with corona pretreatment.
It remains desirable to have a multi-layer, metallized polymeric film having improved bonding of the metal layer to the skin layer, including improved craze resistance, and improved barrier properties, as compared to prior art metallized polymer films.